1. Technical Field of the Invention
The present invention relates to a method for making a potassium niobate thin film, and to a surface acoustic wave element, a frequency filter, a frequency oscillator, an electronic circuit, and an electronic apparatus.
2. Description of the Related Art
As communication technology is remarkably developing with a particular emphasis on mobile communications such as cellular phones, demands for surface acoustic wave elements are rapidly growing. The development of surface acoustic wave elements is proceeding toward miniaturization and an increase of efficiency and frequency. Also, in order to put the surface acoustic wave elements into practical use, the elements need to have a higher electromechanical coupling coefficient (hereinafter expressed as K2), stable characteristics with temperature changes, and a higher propagation speed of surface acoustic waves.
For example, when a surface acoustic wave element is used as a high-frequency filter, a high K2 is desired from the viewpoint of achieving a low insertion loss and obtaining a wide pass band. Also, in order to increase the resonance frequency, it is desired to use a material capable of achieving a high sound velocity for the element from the viewpoint of the possibility of interdigital transducer (hereinafter referred to as IDT) designs. In order to stabilize characteristics at working temperatures, the temperature coefficient of the center frequency (hereinafter referred to as TCF) must be small.
A conventional surface acoustic wave element substantially consists of a piezoelectric single crystal having an IDT thereon. Exemplary piezoelectric single crystals include rock crystal, lithium niobate (LiNbO3), and lithium tantalate (LiTaO3). On the other hand, a cut angle leading to a high K2 has recently been found in a potassium niobate (hereinafter referred to as KNbO3) single crystal. According to a report in Electron. Lett. Vol. 33 (1997), pp. 193–194, a 0° Y-cut, an X-propagating KNbO3 single crystal plate has a K2 of 0.53, showing a possibility of much higher K2 than the piezoelectric single crystals used for the conventional surface acoustic wave element.
The characteristics of the surface acoustic wave element using a piezoelectric single crystal substrate, such as K2, TCF, and sound velocity, are proper values the substrate material has, and they are determined by the cut angle and the propagation direction. Although the 0° Y-cut, X-propagating KNbO3 single crystal plate has a high K2, it does not exhibit the zero-temperature characteristics that a 45°–75° rotated Y-cut, X-propagating KNbO3 single crystal plate shows, at room temperature. Also, the propagation speed is lower than that of strontium titanate (hereinafter expressed as SrTiO3) and calcium titanate (CaTiO3), which are perovskite oxides as with KNbO3.
It is therefore difficult to achieve satisfactory characteristics, such as high K2, high sound velocity, and zero-temperature characteristics, by using only KNbO3 single crystal plate as the piezoelectric layer of the surface acoustic wave element. Accordingly, it is expected that each characteristic above will be enhanced by depositing a KNbO3 thin film on a substrate and controlling the thickness of the KNbO3 thin film. Preferably, the KNbO3 thin film is a closely packed, flat epitaxial film oriented in an optimum direction from the viewpoint of achieving a satisfactory K2 and temperature characteristics. For example, by using a SrTiO3 (100) or SrTiO3 (110) single crystal as the substrate, a 0° Y-cut, X-propagating KNbO3 thin film having a K2 of about 0.5 or a 90° Y-cut, X-propagating KNbO3 thin film having a K2 of about 0.1 may be produced, respectively.
However, since the KNbO3 has a orthorhombic crystalline structure and exhibits K2 anisotropy in the a, b, and c-axis directions, the K2 of the KNbO3 thin film decreases due to the mixture of the directions, even though the KNbO3 is epitaxial. In order to solve this problem, Japanese Unexamined Patent Application Publication No. 11-116397 has disclosed a (020)-oriented perovskite potassium niobate thin film and a surface acoustic wave element including the thin film. In this disclosure, the (020)-oriented perovskite potassium niobate thin film is produced by applying an electric field for polarization that changes the orientation, and thus the piezoelectric characteristics thereof are enhanced.
However, the related art has certain problems s described below.
In Japanese Unexamined Patent Application Publication No. 11-116397, the polarization is performed in such a manner that the (020)-oriented perovskite potassium niobate thin film is subjected to application of a direct electric field to be polarized while being immersed in an insulative liquid, such as silicone oil, to prevent atmospheric discharge, and being heated to 150 to 200° C. After the thin-film is cooled with the electric field maintained, the electric field is removed. However, since the polarization process is performed separate from the manufacturing process of the surface acoustic wave element, it is expensive in time and effort. Also, the detailed technique for applying the direct electric field, including the structure of the electrodes for applying the direct electric field has not been disclosed.
In view of the foregoing disadvantages, one object of the present invention is to provide a method for making a potassium niobate thin film in which, in the process of manufacturing a surface acoustic wave element, a conductive thin film included in the surface acoustic wave element is used for polarization as an electrode for applying an electric field to the potassium niobate thin film that is to serve as the piezoelectric layer of the surface acoustic wave element. Another object of the invention includes providing a surface acoustic wave element, a frequency filter, a frequency oscillator, an electronic circuit, and an electronic apparatus.